Electrocleaning process



Patented Feb. 21, 1933 UNITED STATES PATENT OFFIC THOMAS E. DUNN, OF .BRIDGEPORT, CONNIEIO'JYICIIIEl. ASSIGNOB TO THE BULL-ADD COMPANY, A. CORPORATION 01 CONNECTICUT ELEG'I'BOCLEANILN'G PROCESS No Drawing.

This invention relates to an electro-cleaning rocess for cleaning metal and has for its ject to provide a process for removmg the surface scale and other objectionable 5 matter from forgings, castings, or the like.

It is among the prime objects of the invention to provide a cleaning process which, while being highly eflicient and effective, will avoid the inherent disadvantages of present day mechanical, chemical or electro-chemical processes, such as their injury to the surface of the base-metal ofthe object cleaned, and the expense and time consumed by such operations.

A further object is to provide a process which is particularly adapted for the descaling of objects having recessed or intricate surface designs and which in addition to removing scale or surface dirt from the work, will act to prevent hydrogen embrittlement and which will protect the work against deleterious chemical action, such as pitting or etching, and which will also protect the articles against subsequent rapid surface deterioration.

One highly important object and one which has not been heretofore achieved is to rovide a process which in addition to desc ing of the work will provide a highly tenacious and closely adherent metallic protective film thereover which is joined to the work surface by an alloy-like bond. Among the many advantages of this type of film bond in addition to the natural lasting virtue of such adherence is the translucent characteristic of the film in that defects of the work surface can be observed therethrough. Another advantage of this type of bonded film is the provision of a surface suitable for the reception of permanently adherent subsequent coatings or deposits. I

Another highly important object of the present invention is to provide an electrolytic descaling process of the character disclosed,

the electrolyte of which willremain eflicient Application filed, Iunc 20,

REISSUED and effective over a long period of useful- 1982. Serial R0. 618,403.

ness whereby the process is less vulnerable to interference by ions foreign to those useful to the process. Since with similar processes the electrolyte becomes contaminated by a growing concentration in the'electrolyte of ferric ions so that the efliciency of the process is materially reduced, it is amongthe objects of the present invention to provide a process in which the oxidation of ferrous ions to ferric ions is partially or wholly inhibited.

Attention is directed to applicants Patent No. 1,775,671, granted September 16, 1930, which discloses a process for acid cathodic descaling and simultaneous protection of 80 forgings, castings, or similar scaled metallic articles. This application is a joint continuation of applicants pending application No. 451,517, filed May 10, 1930 and applicants joint application N 0. 482,151, filed September 66 15, 1930 and is, therefore, therewith a continuation in part of the application which resulted in applicants prlor patent above enumerated, the claims of which broadly cover a process which involves the genera- 70 tion of excessive hydrogen at the work which acts to mechanically detach the scale from the work surface, and in which the descaling of any portion of the work surface is followed instantaneously by" the protection of the pilleaned surface by the deposit of a metal The present process has among its objects to partake of the desiderata of the process of the above patent, outstanding among which are that the finished work is (1) free from pits or etches, (2) free from voltaic couples and (3) free from hydrogen embrittlement.

In addition to these virtues, the present rocess combines with them the advantage 0 producing a less soluble and more durable protecting film having a lighter and brighter appearance and which is thus more pleasing than the opaque lead film referred to in my patent. The film prod lQQd by the present process is not only harder and free from smut, but is bonded more adherently to the surface, its color and adherence being a valuable aid to inspection of the underlying body as well as providing a more presentable and thus more salable article and one readily adapted to receive subsequent permanently adhering coatings produced by hot tinningor galvanizing. Another novel feature of this process is that the film produced is susceptible of alloy union with the base metal as distinguishing from lead which will not alloy with iron. Thus the film, if not alloyed at the time of deposit, will so unite with the work as a result of subsequent heat application. The present process also has the advantage of retarding the increase of ferric ion concentration of the electrolyte, thus insuring a longer and/or more highly efiicient life for the bath.

In carrying out the present process I make the work, which is preferably iron or steel, cathode in an acid bath which contains ions of a metal having an afiinity for iron and which will upon deposit wet the iron surface, and thus closely adhere thereto. Tin is utilized as a preferable illustration of the metal to be selected, although suchmetals as zinc and its sister metal, cadmium, may be used since they display a similar affinity, and thus will wet and alloy with the work surface. The acid solution may be composed of sulfuric acid, hydrochloric acid, and common salt. However, I prefer an electrolyte having little or no chlorides therein. Since the virtue of the chloride ions resides chiefly in their effectiveness to cause accelerated anode corrosion and since with the use of tin, anode {corrosion takes place without the presence of chlorides, and further, since chlorides attack heating coils used to warm the electrolyte and other accessories, they are preferably avoided. Such chloride free electrolytes are also cheaper and are simpler to compound. Therefore, I prefer to use the following electrolyte:

10% H by volume, 66 B.

3 oz. per gal. of sodium sulphate (Na SO 10 H O) While this composition illustrates one possible electrolyte, it will, of course, be understood that difi'erent components may be used and widely varying proportions substituted. Thus, when using zinc I prefer to substitute sodium or mercuric chloride for the sodium sulphate. The wide departure from the specified formula to which the process is susceptible has a distinct advantage in that it does not require the close chemical control, which is advisable in other processes.

While the process will function properly by the introduction of the ions of the metal to be deposited through the inclusion in the electrolyte of salts thereof, I prefer for the quired for the suflicient excitation of the bath is preferably formed of insoluble anodes. A further virtue of the use of a large area of insoluble anodes is that since no energy is required to disintegrate and ionize metal at the surface of such insoluble anodes, it follows that for a given energy input, the volume of hydrogen liberated at the cathode is increased and consequently the efficiency of the process is greater than when all the anode area is formed of the metal used for the protecting film. In carrying out the process, I prefer to use cathode current densities approximately 50-100 amps. per square foot of work surface under treatment.

It has been found that the efficiency of the process decreases at a substantially inverse ratio to the increase of ferric ion concentration. Therefore, the present invention embraces the use of anodes at which reactions tend to inhibit the oxidation of the ferrous ions to ferric ions. The electrolyte of the present process has thus been found to have materially longer and/or more highly efficient life than the electrolyte of a process using lead. This advantage in the present process is deemed to be the result not of differences in electrolytes but as a result of marked differences in the anode reactions of this process. Theoretically, among these differences is that in the use of this process, a colloidal shielding film of metastannic acid is formed on the insoluble anode surface.

This film prevents the electrolyte bearing ferrous ions from coming into direct con-' tact with the surface of the anode where the ferrous ion content would be actively oxidized to the ferric state. However, when lead is used as the protective metal film and the anode surface is composed of lead or a lead alloy, no such shielding diaphragm-like film is formed and consequently the electrolyte bearing ferrous ions readily come into direct contact with the surface of the lead anode. Consequently, the ferrous ions are rapidly oxidized to the ferric state. Furthermore, the surfaces of lead anodes become coated with lead peroxide (PbO which, as is well known, exerts a strong catalytic action favoring oxidization and therefore, with lead anodes, two distinct anode conditions rapid increase of the ferric D. concentrationof the electrolyte. This is in shar distinction to the highly favorable an e reaction of this present process, which leads to the formation of an enshroudin metastannic colloidal diaphragm-like film w ich tends to inhibit the oxidation of the ferrous ions.

When a scaled article is introduced in the electrolyte as cathode and is subjected to high current densities, excessive quantities of hydrogen are vigorously liberated at the surface of the work. The action of the hydrogen, either along, or in conjunction with, chemical attack, acts mechanically to wedge or crowd off the scale, leaving a denuded clean surface area. Since the clean surface provides a direct relatively less-resistant electrical path through the base metal, the deposit of a metal film such as tin, will take place immediately thereon, and since such deposit acts to increase the polarization at that portion of the surface, the concentration of electrolytic action will shift or be thrown to other less polarized portions of'the surface. This so-called throwing characteristic of the process has, in connection with the use of metals, such as tin, having lower over-voltage than lead and having an afiinity for iron, a distinct difference from the reaction of lead ions at the cathode. Thus, while it would appear from a consideration of throwing power alone that such meta-ls would be less eflicient than lead, it will be apparent that less energy is required at the cathodes for their deposition, because their hydrogen overvoltage is less, and the natural afiinity of tin or like metals for iron will also aid in the reaction and in practice tends to offset the lower throwing power.

In the operation of the process, it is found that the initial stratas of the film deposit actually tend to wet or alloy with the iron surface, the result being that in place of the deposit of a mechanically bonded and thus less adherent lead film, there is a positive tendency of the tin molecules to wet and spread over the surface of the base metal and to join therewith by metallurgical union. Another feature of the process is that the film is of a metal which will readily form a true alloy with the base by subsequent heatmg.

The process, therefore, produces a bimetallic article, the outer surface of which is an integral portion of the whole and yet formed of a. different metal. It will, of course, be understood that with the use of the high current densities specified, there may be deposited on top of the alloy-like film, loose, spongy deposits of the pure metal such as tin or zinc. This may be washed or brushed off, leaving a very thin film which has the characteristics of translucency in that flaws of the base metal such as hair line cracks may be observed therethrough. Furthermore, the finished article resents a pleasing bright surface which is iiighly advantageous in certain fields. This surface has a high scratch hardness and is free from the graphite-like soiling characteristic of lead surfaces. The integral bonding of the film is also of ma or importance in connection with subsequent treatments, such, for instance, hot tinning or galvanizing.

From a consideration of this specification, it will be understood that this process is not restricted to the use of the specific metallic ions referred to, but is understood to be generic in its teachingof the use of ions having an afiinity for or being capable of alloy bonding with the base metal of the work and to metals which tend to inhibit the oxidation of ferrous to ferric ions in the electrolyte. As indicated, the characteristics of the bath, not only as to the proportions of the ingredients, but as to the ingredients themselves, can be widely varied without departure from the spirit or scope of the invention.

Having thus set forth the nature of my invention, what I claim is:

1. A method of cleaning surfaces which includes the step'of removing surface f0reign matter by the cathodic production of hydrogen at the work and simultaneously protecting the cleaned surface by the deposit thereon of tin in an acid electrolyte in which there are soluble and insoluble anodes.

2. A method of cleaning surfaces which includes the step of rcmovingsurface foreign matter by the cathodic production of hydrogen at the work and simultaneously protecting the cleaned surface by the deposit thereon of tin in an acid electrolyte in which thereare soluble and insoluble anodes, the former giving to the electrolyte ions capable of depositing on the cleaned work simultaneously with the cleaning.

3. A method of cleaning surfaces which includes the step of removing surface scale by subjecting the work to cathodic action in an acid electrolyte in which there are soluble and insoluble anodes the former including tin and providing the requisite surface area for continuously maintaining a desired tin ion concentration for simultaneously depositing during the cleaning, a metallic protective film on the work.

4. A metal cleaning process which includes the step of removing surface foreign matter by the cathodic production of hydrogen at the work and simultaneously protecting the cleaned surface by the deposit of tin thereon in an acid electrolyte containing salts of tin which by the action of the process automatically forms a shielding diaphragm of meta stannic acid about an insoluble anode to inhibit the oxidation of low valence ions which may be formed from dislodged foreign matter.

5. A process as set forth in claim 4 characterized by the use of a soluble anode including tin to continuously supplyingtin ions to the solution.

5 6. A process as set forth in claim 4 characterized by the use of both soluble and insoluble anodes the former providing tin for the deposit on the work and for the formation of the metastannio acid film.

Signed at Bridgeport in the county of Fairfield and State of Connecticut this 16th day of June, A. D. 1932.

THOMASE. DUNN. 

